Abstract

Single-particle dynamics in simple models for the static reversing magnetic field of the geotail current sheet have been extensively studied in the case where the reversing field B-x(z) varies, the linking field B-z is constant, and the crosstail field B-y is zero (and some generalization to include time dependence has been achieved). More recently, numerically integrated trajectories in static reversals which include a constant shear B-y component have suggested some differences in the nature of the dynamics in this and the B-y = 0 case. The invariant of the z cross-sheet motion for the B-y = 0 case is well known, here we find its equivalent for systems with constant B-y. Our results hold for reversals with general z dependence and arbitrary constant B-y. The form of this invariant suggests that it is still conserved for trapped particles, but for certain values of energy, B-y and B-z, the invariant is destroyed and particles are detrapped. This corresponds to an increase in the volume in phase space available to current carrying particles that transit the sheet. For typical magnetotail parameters, both protons and electrons in an average 1 R-E thick sheet will be detrapped, but in a thin similar to 100 km sheet protons will not.